Tool Class

• java.lang, automatically imported
• java.util
• java.io
• java.awt, javax.swing, GUI
• java.net
• java.sql

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Object class

1. equals()
   • == is reference equal
   • equals() is content; one.equals(another)
   • If override equals(), need to override hashCode()
2. getClass()
   • a final method, cannot be overloaded
3. toString() return a string of the object
4. finalize()
5. thread: notify(), notifyAll(), wait()

Wrapper

Wrapper for primitive data type to ref type, like Integer

1. Constant: Integer.MAX_VALUE, Double.NaN, Double.POSITIVE_INFINITY
2. valueOf(String), toString()
   • convert
3. get value: xxxxValue()
   • intValue() for Integer
4. value is immutable
5. Override toString(), equals()
6. Double: parseDouble(), max, min …

Boxing and Unboxing

• Integer I = 5;
  • I = Integer.valueOf(5);
• int i = I;
  • i = I.intValue();

Math

• E
• PI
• abs()
• floor()
• exp()
• remainder(double f1, double f2)
• log()
• min(float a, float b)
• random(): 0 - 1
• rint(): 4.4 to 4; 4.51 to 5
• sqrt()
• sin(), cos(), tan(), asin(), acon(), atan()

System

• System.getProperty(String name)
• System.getProperties()
• add property while running java -D varName = varValue MyProg

String

• String class is immutable
• StringBuffer, StringBuilder can be mutable

Ops:

• Return a new instance: concat, replace, replaceAll, substring, toLowerCase, toUpperCase, trim, toString
• Search: endsWith, startsWith, indexOf, lastIndexOf
• Compare: equals, equalsIgnoreCase
• charAt(pos), length
• format: %1$,8.5f %No$sign...

StringBuffer

Constructor

• StringBuffer()
• StringBuffer(int capacity)
• StringBuffer(String initialString)

Methods

• append
• insert
• reverse
• setCharAt
• setLength

String split

• java.util.StringTokenizer
• Constructor: StringTokenizer(String str, String delim);

Method:

• public int countTokens(): no. of splits
• public boolean hasMoreTokens()
• public String nextToken()

Date

1. Calendar
   • get an instance: Calendar.getInstance()
   • get(DAY_OF_MONTH), .getDisplayName(DAY_OF_WEEK)
   • .set, .add(HOUR,1), .roll(MONTH,5)
   • .setTime(date), .getTime()
   • use fields and methods directly: import static java.util.Calendar.*;
2. Date
   • new Date(), new Date(System.currentTimeMillis())
   • .setTime(long), .getTime()
3. SimpleDateFormat("yyyy-MM-ddHH:mm:ss")
   • .format, .parse

Time

• java.time.*
• java.time.format.*

Class:

• Instant
• Clock
• Duration
• LocalDateTime
• LocalDate
• LocalTime
  • .of, .parse, .format, .plus, .minus

Collection API

3 interface, w sub interfaces:

• Collection
  • List: remember order, allow duplicates
  • Set: no order, not allow duplicates
• Map
  • key-value pair

Methods:

• add(element : Object) : boolean
• remove(element : Object) : boolean
• size() : int
• isEmpty() : boolean
• contains(element : Object) : boolean
• iterator() : Iterator

Structure:

• Collection
  • Set
    • HashSet
    • TreeSet
  • List
    • Vector
      • Stack
    • ArrayList
    • LinkedList
  • Queue
    • LinkedList
    • PriorityQueue
    • ArrayQueue

List

Linear list: List<Type> name = new LinkedList<>();

Class:

• ArrayList
• LinkedList

Interface:

public interface List<E> extends Collection<E> {
    E get(int index);
    E set(int index, E element);
    void add(int index, E element);
    E remove(int index);
    int indexOf(Object o);    
}

Iterator

All Collection can generate.

• Iterator iterator= iterable.iterator();
• while( iterator.hasNext()) doSomething( iterator.next());
• equivalent to enhanced for

Stack

• LIFO
• public Object push(Object item)
• public Object pop()
• public boolean empty()
• Stack<Type> name = new Stack<>();

Queue

• FIFO
• LinkedList
• Insert: add(e), offer(e) return value
• Remove: remove(), poll() return value
• Examine: element(), peek() return value

Set

• HashSet
• TreeSet

No duplicates

• hashCode() not the same
• If same, use == and equals() having unequal results

Map

Can get sets:

• entrySet()
• keySet()
• values()

Realization:

• HashMap
• TreeMap

Ops:

• Map<String,String> map = new TreeMap<String,String>();
• .put(key, value)
• .get(key)
• entry.getKey()
• entry.getValue()
• .keySet()
• .entrySet().iterator()

Sort

• Arrays class can be used to sort and search array
  • Arrays.asList(10, 7, 6, 5, 9) return a list object
  • sort(), binarySearch(); do sort() first before binarySearch()
    • Arrays.<String>sort(s);
• Collection
  • Colletions.sort( school, new ComparatorName())
  • or Colletions.sort( school, (p1,p2)-> p1.age-p2.age)

Generic

Must be ref type

Class

import java.util.*;

class GenericTreeClass {
	public static void main(String[] args){
		TNode<String> t = new TNode<>("Roo");
		t.add("Left"); t.add("Middle"); t.add("Right");
		t.getChild(0).add("aaa");
		t.getChild(0).add("bbb");
		t.traverse();
	}
}

class TNode<T>
{
  private T value;
  private ArrayList<TNode<T>> children = new ArrayList<>();

  TNode(T v) { this.value = v; }
  public T getValue() { return this.value; }
  public void add(T v) {
    TNode<T> child = new TNode<>(v);
    this.children.add(child);
  }
  public TNode<T> getChild(int i) {
    if ((i < 0) || (i > this.children.size())) return null;
    return (TNode<T>)this.children.get(i);
  }

  public void traverse() {
    System.out.println(this.value);
    for (TNode child : this.children)
      child.traverse();
  }
}

Method

import java.util.*;

class GenericMethod {
	public static void main(String[] args){
		Date date = BeanUtil.<Date>getInstance("java.util.Date");
		System.out.println(date);
	}
}

class BeanUtil{
	public static <T> T getInstance( String clzName ){
		try
		{
			Class c = Class.forName(clzName);
			return (T) c.newInstance();
		}
		catch (ClassNotFoundException ex){}
		catch (InstantiationException ex){}
		catch (IllegalAccessException ex){}
		return null;
	}
}

Limit on type

• ?: all: reverse(List<?> list)
• extends: addAll(Collection<? extends E> col)
• super: fill(List<? super T> list, T obj), fruit for the list, can have apple.. in it

Popular algorithms

Exhaust algo, brute force

for(;;){ if(); }

Iterative algo

while() { x = f(x); }

Recursive algo

f(n){f(n-1);}

Back-track

x++; if(...) x--;